3 Answers
3

First thing I'd check is that it's got the AWD driveline - there should be a driveshaft from the gearbox in the front connecting to a diff at the rear "axle", plus driveshafts from each rear wheel to the diff. If that's missing, well, there's your problem...

Assuming it has an AWD driveline, my suspicion would be that the center diff that is supposed to feed power to the rear wheels isn't working as it should. A lot of the more recent FWD-based cars normally run something like a 90/10 front rear split and only feed more power to the rear wheels if the front wheels have traction issues. Actually, according to this review of a 2006 XC90 the split is 95%/5% under normal circumstances and can change up to 50%/50% depending on conditions.

FWIW, I've driven an AWD Volvo XC90 several times before and that was my experience as well. If that's normal, then it's the worst AWD system I've ever driven. Had terrible torque steer under hard acceleration, and front wheelspin was a major issue on loose surfaces. Didn't think much about it (even though it was new when I first drove it, and continued to drive it occasionally until it was about 2 years old) since it wasn't my vehicle...

Note that this won't work for all AWD systems. For example, a torsen requires there to be SOME friction on both of it's output shafts for it to work, and all 4 wheels may not spin if the front and rear differentials aren't locking differentials...
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Sean ReifschneiderSep 23 '11 at 4:02

1

Specific case this fails is on the AWD Mitsubishi Eclipse. Most of them were built with an open front differential (I've heard rumors of one with an LSD front diff, but have never found one myself). The rear differential is viscous coupling, so even there you have to get some good spinning before it'll hookup the other side.
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Brian KnoblauchSep 23 '11 at 12:17